1. Field of the Invention
The present invention relates to a rotary encoder which detects a rotation angle.
2. Description of the Related Art
Previously, a rotary encoder has been used for measuring an angle in a machine tool, an FA apparatus, or the like. Types of detecting the angle in the rotary encoder are roughly divided into an incremental type and an absolute type. Japanese Patent Laid-Open No. 2006-214929 discloses an incremental-type rotary encoder. The incremental-type rotary encoder can be easily manufactured, but there is a problem that angle information is deleted if the electric power is turned off or errors are accumulated by an exogenous noise. On the other hand, an absolute-type rotary encoder has advantages that it is highly accurate since it does not accumulate errors and that the movement to a home position is not necessary even if the electric power is turned off.
Japanese Patent Laid-Open No. 2007-147465 discloses an absolute-type rotary encoder. The rotary encoder uses an angle detection method called vernier in which a phase difference between a plurality of periodic signals having different periods from each other is calculated to obtain a periodic signal that has a period different from each of the original periods.
In the previous vernier detection method, a relative phase between original periodic signals is changed when a scale is decentered with respect to a rotary shaft of the scale. If the decentering of the scale is large, a detection error of an angle signal obtained by the vernier detection increases and phase synchronization with a lower periodic signal cannot be ensured. Therefore, when the scale is fixed to the rotary shaft, a highly accurate adjustment is required and it prevents reducing the cost.